This laboratory discovered the photoactivation of urocanase of Pseudomonas putida and described the characteristics of this photoactivation and its action spectrum. We have identified the chromophore, elucidated the nature of the dark reversion, and described the chemistry of the photoactivation. The principal hypothesis: Photoactivation of urocanase is the photodissociation of the bound NAD-sulfite adduct; dark inactivation is the addition of the nucleophile, sulfite, to the coenzyme, NAD ion. Evidence to support this hypothesis is incomplete and more studies are needed. Some objectives are to: 1) determine the quantum yield; 2) determine the stoichometry of the sulfite-enzyme reaction; 3) account for the increase in sulfhydryl groups measured upon photoactivation; 4) account for changes in stability upon photoactivation; 5) study structural features of the enzyme by tryptophan and N-terminal amino acid analysis, chemical modification, and electrophoresis; 6) identify the nucleophile that brings about the dark reversion in vivo. Procedures for photo-chemical studies involve spectrophotometry, measurement of fluence by radiometry and actinometry, and monochromatic radiation. Stoichometry studies involve radioactive labeling of urocanase by (35S) sulfite and assay by scintillation counter. The second hypothesis: The substrate binds to the enzyme at the NAD ion to form an NAD-urocanate adduct. We propose to demonstrate this complex with the help of inhibitors that do not affect binding, with a stop flow study, or with a low temperature study with purified liver enzyme. A closely related objective is to demonstrate a proposed NAD-produce adduct. NAD ion adducts are revealed by appearance of absorption bands at 300-360 nm. In conclusion, we reported a new photochemical reaction in the photoactivation of urocanase and propose to describe this system in more detail.